Fast continuously wavelength tuning single frequency fiber laser using tunable polymer optical filters

ABSTRACT

A method for generating a laser projection by employing a laser gain medium for receiving an optical input projection from a laser pump. The method further includes a step of employing a mode selection filter comprising an electro-optical (EO) tunable layer disposed between two parallel reflection plates for generating a laser of a resonant peak.

This Formal Application claims a Priority Date of Oct. 14, 2003 benefitfrom a Provisional Patent Applications 60/510,133, a Priority Date ofOct. 17, 2003 from Provisional Application 60/511,681, and Apr. 12, 2004benefited from Provisional Applications 60/560,983 filed by the sameApplicant of this Application respectively.

FIELD OF THE INVENTION

The present invention relates generally to apparatuses and methods forproviding single frequency laser sources. More particularly, thisinvention relates to new configurations and methods for providingcompact single frequency fiber laser with optical sensing suitable forimplementation coherent communication, laser tracking, coherentdetection laser radars and instrument.

BACKGROUND OF THE INVENTION

Conventional technologies of tunable lasers are confronted withtechnical difficulties and limitations. Specifically, tunable laserswhich are implemented with mechanical or temperature tuning to changethe cavity length to tune the wavelength do not provide stable andaccurate turning in terms of power and frequency. More details of suchtechnical limitations are more fully discussed in references such as“Stress induced tuning of a diode laser excited monolithic Nd:YAGlaser,” by Adelbert Owyoung and Peter Asherick, in Opt. Lett. 12(12),999-1001 (1987); and “Efficient, frequency stable laser diode pumpedNd:YAG laser,” by Bingkun Zhou, T. J. Kane, G. J. Dixon, and R. L. Byer,in Opt. Lett. 10(2), 62-64 (1985).

On the other hand, different approaches using tuning grating forfrequency tuning of linewidth not very narrow, acoustic optical tuningfilter (AOTF), or a Fabry Perot (FP) cavity can provide stableoperation. However, these tuning methods cannot provide continuoustuning and accurate access of the wavelength due to the difficulties ofhysterises. More details about the discussions of the hysterises aredisclosed in several references. These references are: Hidemi Tsuchida,“Tunable, narrow linewidth output from an injection locked high powerAlGaAs laser diode array,” Opt. Lett. 19(21), 1741-1743 (1994); JianLiu, Stable and high speed full range laser wavelength tuning withreduced group delay and temperature variation compensation, PatentApplication Number 10/337081, January 2002; and M. Auerbach, et al., “10W widely tunable narrow linewidth double clad fiber ring laser,” OpticsExpress 10(2), 139-144 (2002).

In the meantime, continuously frequency-tunable single frequency laserswith linewidth in the order of kHz are important to coherent opticalcommunications, coherent laser radars, optical sensing, test andmeasurement, and laser tracking of flying objects. For these reasons,there are urgent demand to develop a laser system that the wavelengthcan be continuously tuned and randomly accessed with excellentwavelength accuracy, stability, and linewidth.

A co-pending Application 10/337,081 filed by the Applicant of thisinvention is hereby incorporated by reference. In this co-pendingApplication, a single frequency fiber laser to provide laser output ofsharp and stable highly defined frequency is disclosed. Meanwhile, thereis still a need to provide further improvement on the continuouslytunable single frequency lasers with fine linewidth tuning accuracy.

Therefore, a need still exists in the art of fiber laser source designand manufacture to provide a new and improved configuration and methodto provide continuously frequency-tunable single frequency lasers withlinewidth in the order of kHz such that the above discussed difficultymay be resolved.

SUMMARY OF THE PRESENT INVENTION

It is therefore an object of the present invention to provide acontinuously frequency-tunable single frequency lasers with linewidth inthe order of kHz such that the above described difficulties encounteredin the prior art can be resolved.

Specifically, it is an object of this invention to provide a new ways ofimplementation of continuously frequency tunable filter with ahigh-speed electro-optical tunable filter serving as a mode selectionfilter to work with a bandpass filter. In a preferred embodiment, themode selection filter includes an EO tunable polymer with electrodes toapply voltage for frequency tuning. In an alternate preferredembodiment, the electrodes applied to the polymer material are along atransverse direction. Instead of perpendicular to the direction of lightpropagation, the electrodes are parallel to the light propagationdirection. The polymer can be EO or TO polymers. Other material havingsimilar EO or TO properties may also apply to this invention. Thepositions of the polymer cells can be in the middle, filled in the FPcavity, or on one side of the cavity.

Briefly, in a preferred embodiment, the present invention discloses asingle frequency fiber laser that includes a laser gain medium forreceiving an optical input projection from a laser pump. The fiber laserfurther includes a mode selection filter comprising an electro-optical(EO) tunable layer disposed between two parallel reflection plates forgenerating a resonant peak. In a preferred embodiment, the modeselection filter further includes two electrodes for applying a tuningvoltage to said EO tunable layer.

In essence this invention discloses fiber laser that includes a modeselection filter combining with a bandpass filter for generating acontinuously tunable single frequency tunable over a specified frequencyrange.

In a preferred embodiment, this invention further discloses a method forgenerating a laser projection by employing a laser gain medium forreceiving an optical input projection from a laser pump. The methodfurther includes a step of employing a mode selection filter comprisingan electro-optical (EO) tunable layer disposed between two parallelreflection plates for generating a laser of a resonant peak. In apreferred embodiment, the method further includes a step of projectingthe laser of the resonant peak through a bandpass filter for generatinga laser of substantially a single frequency. In another preferredembodiment, the method further includes a step of applying a tuningvoltage to the EO tunable layer for tuning a frequency of the fiberlaser. In another preferred embodiment, the step of employing a modeselection filter further comprising a step of disposing a LiNbO3 as theEO tunable layer between the parallel reflection plates. In anotherpreferred embodiment, the step of employing a mode selection filterfurther comprising a step of disposing a semiconductor as the EO tunablelayer between the parallel reflection plates. In another preferredembodiment, the step of employing a mode selection filter furthercomprising a step of disposing a electro-optical polymer as the EOtunable layer between the parallel reflection plates. In anotherpreferred embodiment, the step of employing a mode selection filterfurther comprising a step of disposing a PLZT as the EO tunable layerbetween the parallel reflection plates. In another preferred embodiment,the step of employing a mode selection filter further comprising a stepof disposing a KTN (KTaNbO3) as the EO tunable layer between theparallel reflection plates.

In a preferred embodiment, this invention further discloses a modeselection filter that includes an electro-optical (EO) tunable layerdisposed between two parallel reflection plates for generating aresonant peak. In a preferred embodiment, the mode selection filterfurther includes two electrodes for applying a tuning voltage to the EOtunable layer. In another preferred embodiment, the mode selectionfilter further includes two electrode plates and the EO tunable layerbetween the parallel reflection plates disposed between the twoelectrode plates. In another preferred embodiment, the EO tunable layerbetween the parallel reflection plates filling a space between theelectrode plates. In another preferred embodiment, the EO tunable layerbetween the parallel reflection plates occupying partially a spacebetween the electrode plates. In another preferred embodiment, the EOtunable layer between the parallel reflection plates attached to one ofthe electrode plates. In another preferred embodiment, the electrodesare transparent electrodes. In another preferred embodiment, the EOtunable layer between the parallel reflection plates comprising LiNbO3.In another preferred embodiment, the EO tunable layer between theparallel reflection plates comprising a semiconductor. In anotherpreferred embodiment, the EO tunable layer between the parallelreflection plates comprising an electro-optical polymer. In anotherpreferred embodiment, the EO tunable layer between the parallelreflection plates comprising a PLZT. In another preferred embodiment,the EO tunable layer between the parallel reflection plates comprising aKTN (KTaNbO3).

These and other objects and advantages of the present invention will nodoubt become obvious to those of ordinary skill in the art after havingread the following detailed description of the preferred embodiment,which is illustrated in the various drawing figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is functional block diagram for a tunable single frequency fiberring laser disclosed in a co-pending Patent Application implementing amode selection tunable filter;

FIGS. 2A to 2C illustrate three alternate fast frequency selectiveswitches;

FIG. 3 shows a diagram of the variation of the refractive index asfunction of applied voltage;

FIG. 4 is a diagram for showing the wavelength tuning for 2 mm cavityfilled with a five micron EO polymer with R-0.995;

FIG. 5 is a diagram showing wavelength tuning for a 20 micron EO polymerfilled cavity with R=0.9;

FIG. 6 shows a polymer filled optical tunable filter;

FIGS. 7A and 7B show the relative index change and applied voltageversus spacing for a 30 GHz frequency shift;

FIGS. 8A and 8B are cross sectional views of the structures for EOpolymer tunable filter;

FIG. 9 is a functional block diagram for showing a tunable fiber laserprovided for locking the frequency and stabilizing the power;

FIG. 10 is a flowchart for showing the tuning and locking algorithm fora tunable fiber laser of this invention;

FIG. 11 shows a transmission curve of an etalon for tuning and lockingthe frequency of the tunable fiber laser;

FIG. 12 is a diagram for showing a typical responsivity of InGaAsphotodetector; and

FIG. 13 is a schematic diagram for showing an integrated solution forproviding components used in the tunable fiber laser.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1 for a tunable single frequency fiber ring laser 100implemented with a mode selection tunable filter 140 to select thelasing mode wherein an electrical optical (EO) polymer is used in thefilter to change the phase of the filter at fast speed and highaccuracy. FIG. 1 shows a single frequency fiber ring laser 100 as aunidirectional cavity. The ring laser 100 includes a 980/1480 nm laserpump 110 to transmit a laser through a WDM 125 to a gain medium PMerbium doped fiber (EDF) 130. The laser then projects through a firstisolator 135 to a mode selection filter 140 to select a single modeoperation. The coupler 145 is for outputting a laser output 160 at apre-selected ratio. One or two isolators, e.g., a second isolator 155are used to assure the uni-direction operation.

Referring to FIG. 2A for an exemplary embodiment of the mode selectiontunable filter 140 implemented as a fast wavelength selective switch.The fast wavelength selective switch includes a layer of EO material 170is sand witched between two parallel plates with high reflectance 180.The EO material layer 170 can be inserted as shown in FIG. 2A orattached/integrated as that shown in FIGS. 2B and 2C with the two plates180. Electrodes layer 175 has to be transparent to the operatingwavelength range. This configuration takes advantage of the FP cavityand the fast response nature of EO material to achieve fast wavelengthselective switches.

The EO materials used to fabricate the wavelength selective switches 140could be LiNbO3 or semiconductor or EO polymers or PLZT. An electricaloptical (EO) polymer is a good candidate because it provides with largeEO coefficient and easy fabrication process. It can be tuned at veryhigh speed (110 GHz modulator has been demonstrated, ref. 4). The γ33value of an EO material is an important parameter for EO modulators. Theindex modulation can be represented as:

${\Delta \; n} = {\frac{1}{2}\gamma_{33}n_{0}^{3}\frac{V^{\prime}}{d^{\prime}}}$

where V is the applied voltage. Table 1 shows some EO materialcharacters. The EO polymers have large γ33 values (>40 pm/V) and noconstraints due to lattice match with the substrate and are widely usedfor high speed and low driving voltage optical modulator.

TABLE 1 A survey of EO materials Name γ(pm/V) λ(nm) n LiN_(b)O₃ 28.81300 2.14 KN_(b)O₃ 34 1300 2.12 GaAs 1.2 1020 3.50 FTC/PMMA 83 1300 1.65DRI/PMMA 22 1300 1.58

To achieve fast continuous wavelength tuning and narrow linewidth singlemode operation, the FP cavity of the mode selection has to be designedwith a large spacing and high reflectance. A band pass filter has to beused to reject repeated modes unwanted from the FP cavity. It is anoption to coat the surfaces of the FP cavity with band selectivecoating. FIG. 3 shows simulation results for index change as a functionof applied voltage for EO polymer material under various thicknesses. Itis easy to understand from the equation listed above that thinner layerof EO polymer requires less voltage applied to obtain a given indexchange. A co-pending Patent Application 10/337081 discloses severalexamples on tuning wavelength over a wide range. In addition to achievefine tuning with a narrow band with the bandwidth controlled by thebandpass filter as shown in FIG. 1, FIG. 4 provide an example ofwavelength tuning by changing the refractive index of the cavity spaceas shown in FIG. 2C. The cavity is spaced at a distance of 2 millimeterswith a polymer layer of thickness 5 micrometers. An index change ofapproximately 1.5% can cause >30 GHz wavelength tuning range andachieving nano second (ns) switching time with 5 micron thick cavity.The performance ensures that single frequency operation is achievedwhile tuning for a fiber laser cavity with a couple of meters length.

The system configuration and tuning techniques can be applied to linearconfiguration of fiber laser as well. The gain medium can be any rareearth doped or semiconductor based, such as erbium-doped fiber (1550 nmrange), Tm doped fiber, Tellurite fiber, Yb doped fiber (1064 nm range),Er/Yb doped fiber, semiconductor amplifier. It can be single mode fiber,and double cladding fiber.

Furthermore, if the band pass filter can be tuned across the wholebandwidth of the gain medium, by combining with the continuous tuning ofthe mode selection tunable filter the laser can be tuned continuouslyacross the whole bandwidth at single frequency operation. The tunableband pass filter 150 can be provided with a bandwidth large enough tocover the tuning bandwidth of mode selection tunable filter 140 andnarrow enough to reject other modes of the mode selection tunablefilter. The disclosures for some of the embodiments that cover thescopes have already bee described in the co-pending Patent Application10/337,081, therefore the details will not be repeated here.

By employing the polymer based FP cavity as that shown in FIGS. 2A to2C, fast tuning can be achieved. A cavity of 20 micrometers can be usedto tune the central wavelength by changing the refractive index. Achange of the refractive index by 1.5% can cause a central wavelengthchange of more than 30 nanometers. The reflectance is 90% and thebandwidth is 100 GHz, which is matched with the FSR of the modeselection tunable filter. By combining the two filters, wide rangetuning can be achieved while maintaining single frequency operation ofthe fiber laser. FIG. 5 shows the tuning function while changing theindex of refraction of the EO polymer.

FIG. 6 shows an alternate preferred embodiment that has a differentconfiguration to implement the tunable polymer optical filter 140′. Theelectrodes 175′ applied to the polymer material 170′ are transverse toinstead of perpendicular to the light propagation direction. The polymercan be EO or TO polymers. Other material having similar EO or TOproperties may also apply to this invention. The positions of thepolymer cells can be in the middle, filled in the FP cavity, or on oneside of the cavity.

For the purpose of further improving the performance of the singlefrequency laser, the tuning speed of the mode selection tunable filter140 is improved by optimizing the driving voltage applied to the EOpolymer layer in the FP cavity. It is known that the speed of thefrequency tuning is mainly limited by the spacing of the FP cavity andthat is corresponding to the voltage applied to the cavity. To furtherincrease the speed of the tuning, the voltage tuning range is reducedfrom a few hundreds of volts to a range that is below a couple ofhundreds volts, a more controllable range. Since the voltage applied tothe polymer is to change the refractive index by EO effects, the indexchange is proportional to the voltage applied and inversely proportionalto the spacing or thickness of the polymer layer. With these functionalrelationships, in order to obtain same amount change of the refractiveindex and reduce the applied voltage, the spacing of the polymer layerfor the FP cavity must be reduced.

When the spacing is reduced, the bandwidth of the FP cavity is alsochanged thus affecting the mode selection of the fiber laser. Furtherstudies are performed of the variations of the bandwidth of the FPcavity as a function of cavity spacing and the reflectance of the twoparallel surfaces. It is confirmed that for cavity filter with shorterspacing, the bandwidth can be kept unchanged by employing cavity plateswith higher reflectance. A 99.99% reflectance is practically achievable,e.g., the market available reflecting plates by Forrealspectrum, Inc.,provide such reflectance. Specifically, analyses predict that cavityspacing below 50 micrometers is possible to achieve a bandwidth asnarrow as few hundreds of MHz by employing plates of high reflectance.

For the purpose of achieving 30 GHz tuning range of frequency, furtherstudies are conducted by changing the relative index of a polymertunable filter with various sizes of cavity spacing. FIGS. 7A and 7Bshow the changes of the relative index and the applied voltagerespectively as a function of cavity spacing for a 30 GHz frequencyshift. It shows that cavity of smaller spacing needs less index changeand correspondingly less applied voltage to stimulate the change. Byproviding high reflectance FP cavity, it will be possible to achievefaster tuning speed with low driving voltage, e.g., a driving voltagebelow a couple of hundreds of volts.

FIGS. 8A and 8B show two types of filter structures considered inoptimization of the performance of the tunable filter 200. Thedifference between these two structures shown in FIGS. 8A and 8B is theposition of electrodes and high reflection coating in the cavity. InFIG. 6A, the electrodes 210, such as ITO and gold, are coated on glasssubstrate 220 first. Then multi layers high reflection coatings 230,with reflectance over 99%, are deposited on top of the electrodes 210. Aspacer 240 with precise control of the parallel of the two highreflection surfaces (not shown) is put in between to form a FP cavity.The high reflection coating 230 in FIG. 8B is coated directly onto theglass substrate 220 and disposed between the glass substrate 220 and theelectrodes 210. The EO polymer 250 can be inserted in two ways. Thefirst method includes a step to first spin-coat the polymer 250 on oneof the reflection surface 240. Then, applying a step by poling of thepolymer followed by forming the FP cavity by using a precise spacer 240.The other method includes a step by forming the cavity first, thenfollowed by injecting the polymer 250 in the cavity and uses theelectrodes 210 to carry out the poling. The second method is preferredbecause it provides a more practical way of manufacturing.

Tuning and locking mechanism is important to both power and frequencystability of the tunable fiber-laser over a wide range of temperatureand long-term reliable operations. Specifically, when the tunable fiberlaser operates at a given frequency, it is important to be stable over awide range of temperature and have long-term frequency and powerstability. FIG. 9 shows a configuration to provide an efficient way oflocking the lasing frequency and stabilizing the output power by usingan etalon with a free spectral range of 80-100 GHz. The tunable fiberlaser system 300 includes a tap coupler 310 coupled to an output fiber305 to tap a little portion, e.g., 10% from the output port. The tappedoptical signals are then splitted into two paths via a splitter 320. Oneof the paths 330 feeds into a detector 335 to measure a referencesignal. Another path 340 transmits an optical signal to a seconddetector 345 through an etalon 350. A comparator 360 is then employed tocompare the signal differences between the reference signals with thatfrom etalon. An electronic controller 370 applies the frequency shift toadjust the mode selection tunable filter 140 as shown in FIG. 1. Thereference signals are also fed back for adjustments to improve powerstability by adjusting the pump diode to suppress the power fluctuationsand reduce the RIN.

FIG. 10 shows the tuning and locking process to generate stablefrequency and power from the fiber laser configured as that shown inFIG. 9. The process begins with a step of receiving a frequency-tuningsignal (step 400) and using the frequency-tuning signal to generate avoltage to apply to the filter (step 410). A small portion of the outputsignal is tapped and splitted into a reference signal (step 420) and topass through an etalon (step 430). The reference signal and the signalpassing through the etalon are compared (step 440) to determine afrequency shift (step 450). Based on the frequency shift it isdetermined if the frequency stability criterion is satisfied, and afrequency tuning signal is generated (step 400) and the processes byapplying steps 410 to step 450 are repeated. In the meantime, thereference signal is inputted to a power stability detection circuit todetermine if a power stability criterion is satisfied (step 460). A pumpdiode current is adjusted (step 470) to adjust the laser power if it isdetermined from the reference signal that it is required to reduce thepower fluctuations.

For the purpose of providing improved control over frequency stability,the etalon 350 implemented for tuning and locking as described abovemust have low reflectance, e.g., 60%-80% or low finesse in order toprovide sufficient sensitivity to the slope of the transmission curve todifferentiate the frequency shift below 100 MHz. FIG. 11 is a diagramfor showing an example of such transmission curve of the etalon. Itclearly shows that 30 GHz range of frequency shift can be generated withabout 80% of the power change from the etalon. For 100 MHz controlaccuracy, 0.2% power change detection is required. Assuming 10% of 100mW output is taped out for monitoring and control, the power changecorresponding to 0.2% will be 0.2%×100×50%=0.1 mW. Based on theresponsivity a s that shown in FIG. 12, this converts to 0.1 mA currentchange approximately. It is easily detectable and controllable by usinga commercially available detector, e.g., a detector provided byFermionics Opto-Technology.

FIG. 13 shows an integrated module that includes all passive componentsas described above to an integrated and compact package. This compactmodule 500 for implementing with the fiber lasers includes a WDM coupler(not shown) coupled to a dual-core collimator 510 that includes a pumpreflection coating 515. The module further includes a polymer basedtunable FP filter 520 attached with a narrow bandpass filter 530. Towardthe output end, a free space isolator 540 disposed immediately next to areflection output coupler coating 550 attached to an output dual corecollimator 560. By employing the integrated solution as shown in FIG.13, the gain fiber can be spliced into the module directly without anydifficulty. This in turn enable those of ordinary skill in the art tomake the fiber laser ring cavity in a reasonably short length toincrease the mode spacing for ease of selecting narrow bandpass filters.

Although the present invention has been described in terms of thepresently preferred embodiment, it is to be understood that suchdisclosure is not to be interpreted as limiting. Various alternationsand modifications will no doubt become apparent to those skilled in theart after reading the above disclosure. Accordingly, it is intended thatthe appended claims be interpreted as covering all alternations andmodifications as fall within the true spirit and scope of the invention.

1-82. (canceled)
 83. A laser device, comprising: a laser gain mediumconfigured to produce a first laser beam; a mode-selection filterconfigured to receive the first laser beam and to produce a second laserbeam that comprises a plurality of spectral peaks having first linewidths, wherein two of the plurality of spectral peaks are separated bya frequency difference, wherein the mode-selection filter comprises: afirst reflective plate configured to receive the first laser beamorthogonal to the first reflective plate; an optical material disposedbetween the first parallel reflective plate and the second reflectiveplate; and a second reflective plate parallel to the first reflectiveplate, wherein the first reflective plate, the electro-optical material,and the second reflective plate are configured to filter the first laserbeam to produce the second laser beam exiting the second reflectionplate; and a band-pass filter configured to filter the second laser beamand to produce a third laser beam having one of the plurality ofspectral peaks.
 84. The laser device of claim 83, wherein the laser gainmedium is configured to produce the first laser beam in response to apump laser beam.
 85. The laser device of claim 83, further comprising:an optical sensor configured to output an output sensing signal inresponse to the third laser beam; and a controller configured to controlthe mode-selection filter in response to the output sensing signal toreduce fluctuations in power, frequency, or a combination thereof in thethird laser beam.
 86. The laser device of claim 85, wherein thecontroller is configured to control the electrode in the mode-selectionfilter in response to the output sensing signal to reduce fluctuationsin power, frequency, or a combination thereof in the third laser beam.87. The laser device of claim 83, wherein the optical material disposedbetween the first parallel reflective plate and the second reflectiveplate is an electro-optical material that is configured to shiftfrequencies of the plurality of spectral peaks in response to anelectric filed.
 88. The laser device of claim 87, wherein theelectro-optical material is selected from a group consisting of LiNbO3,a semiconductor material, a PLZT, KTN (KTaNbO3), and an electro-opticalpolymer.
 89. The laser device of claim 87, wherein the electric field issubstantially perpendicular to the first laser beam or the second laserbeam.
 90. The laser device of claim 87, wherein the electric field issubstantially parallel to the first laser beam or the second laser beam.91. The laser device of claim 87, wherein the electro-optical materialis configured to vary the first wavelength by more than 40 pm for eachvoltage produced by the electric field applied across theelectro-optical material.
 92. The laser device of claim 87, wherein theelectro-optical material is configured to vary the one of the pluralityof transmission peaks by more than 20 GHz in response to the electricfield.
 93. The laser device of claim 83, wherein the third laser beamhas a line width narrower than 150 GHz.
 94. The laser device of claim83, wherein the laser gain medium, the mode-selection filter, and theband-pass filter in part form a ring-shaped laser resonance cavity. 95.The laser device of claim 83, wherein the band-pass filter has a secondbandwidth wider than the first bandwidth but narrower than the frequencydifference.
 96. A laser device, comprising: a laser gain mediumconfigured to produce a first laser beam; a mode-selection filtercomprising: a first reflective plate configured to receive the firstlaser beam orthogonal to the first reflective plate; and a secondreflective plate parallel to the first reflective plate, wherein themode-selection filter is configured to filter the first laser beam toproduce a second laser beam exiting the second reflection plate, whereinthe second laser beam comprises at least two spectral peaks separated bya frequency difference and having first line widths; and a band-passfilter configured to filter the second laser beam and to produce a thirdlaser beam having one of the two spectral peaks, wherein the band-passfilter has a second bandwidth wider than the first bandwidth butnarrower than the frequency difference.
 97. The laser device of claim96, wherein the first reflective plate and the second reflective plateare positioned substantially orthogonal to the first laser beam and thesecond laser beam.
 98. The laser device of claim 96, further comprisingan electro-optical material disposed between the first parallelreflective plate and the second reflective plate, wherein theelectro-optical material is configured to shift frequencies of the twospectral peaks in response to an electric filed.
 99. The laser device ofclaim 98, wherein the electro-optical material is configured to vary thefirst wavelength by more than 40 pm for each voltage produced by theelectric field applied across the electro-optical material.
 100. Thelaser device of claim 98, wherein the electro-optical material isconfigured to vary at least the two transmission peaks by more than 20GHz in response to the electric field.
 101. The laser device of claim98, wherein the electro-optical material is selected from a groupconsisting of LiNbO3, a semiconductor material, a PLZT, KTN (KTaNbO3),and an electro-optical polymer.
 102. The laser device of claim 96,wherein the third laser beam has a line width narrower than 150 GHz.103. A laser device, comprising: a laser gain medium configured toproduce a first laser beam; a mode-selection filter configured to filterthe first laser beam to produce a second laser beam, wherein themode-selection filter has a transmission spectrum that includes aplurality of transmission peaks, wherein one of the plurality oftransmission peaks has a first bandwidth at a first wavelength and isseparated from one of its adjacent transmission peaks by a frequencydifference, wherein the transmission spectrum of the mode-selectionfilter is tunable by an electric field, wherein the electric field isconfigured to vary the first wavelength at which the one of theplurality of transmission peaks is located; and a band-pass filterconfigured to filter the second laser beam and to produce a third laserbeam, wherein the band-pass filter has a second bandwidth wider than thefirst bandwidth but narrower than the frequency difference, wherein theband-pass filter is configured to pass the one of the plurality oftransmission peaks at the first wavelength while filtering out othertransmission peaks in the plurality of transmission peaks in the secondlaser beam, wherein the laser gain medium, the mode-selection filter,and the band-pass filter in part form a laser resonance cavity in thelaser device.
 104. The laser device of claim 103, wherein the laser gainmedium is selected from a group consisting of an erbium doped fiber(EDF), a rare earth doped fiber, a Tm doped fiber, a Telluride dopedfiber, a Yb doped fiber, and an Er/Yb doped fiber.
 105. The laser deviceof claim 103, wherein the laser gain medium is configured to produce thefirst laser beam in response to the pump laser beam.
 106. The laserdevice of claim 103, wherein the third laser beam has a line widthnarrower than 150 GHz.